Caution: You can't always trust that GPS device

The distress call came in to Sgt. Christopher Buckley late one night: Two motorists and their families had followed their Global Positioning System (GPS) devices off the main road near Windham, Vt., onto a snowmobile trail. Though the road became narrower and the snow deeper, the two men kept driving.

"Then they ran out of road and they sunk in the snow," Buckley, of the Vermont State Police, says of the February incident.

It's not the first time drivers have blindly followed their GPS units, Buckley says. He's assisted on three rescues since January, and others in his division have logged at least three other instances in which people drove over snowbanks onto unplowed roads and got stuck.

One Ft. Drum, N.Y., soldier drove his car over a snowbank and sank so deeply onto a trail that police needed a snow tractor to haul him out, Buckley says.

"I ask them why and they say, 'That's what my GPS told me to do,' " says Buckley, who late last month issued a press release pleading with drivers to trust their common sense over their navigation devices.

As GPS units and their databases of maps become common features in cars and cellphones, more drivers are encountering unmapped areas and map inaccuracies, says Andrea Barisani, chief security engineer for Inverse Path, a computer software company based in England.

In response, some companies are trying to map these obscure nooks, unpaved roads and trails. Others are asking customers to correct inaccuracies and report changes on their routes.

"All of those maps have very big gaps in them, and there's a whole cottage industry developing that's trying to fill in those gaps," says Marcus Needham, president of Mountain Dynamics, Inc., of Tucson, which launched a GPS device last December with a database that includes 100,000 miles of snowmobile trail maps. The company also mapped 75 North American and 420 European ski resorts.

TomTom, which manufactures a GPS device, has asked its users to make corrections directly on its unit's touch screen, such as editing a street name when they spot a mistake, says Tom Murray, a spokesman for the Concord, Mass., company. TomTom vets the information and updates its database.

Users have made more than 5 million corrections, including hundreds of thousands of street names, since the program launched early last year.

GPS navigating devices are only as accurate as the map data supplied to them, says Jessica Myers, spokeswoman for Garmin, an Olathe, Kan.-based company that makes a popular GPS device.

"Even if you are driving with the most up-to-date map, there is a chance that there could be some data that's not completely accurate," Myers says. "Lots of people have the mistaken notion that there are satellites in the sky that take pictures of the earth as soon as a new cul-de-sac is built in a brand-new neighborhood in a small town, and that that information is just beamed to GPS. That's not the case."